The present invention relates to a transmission apparatus for transmitting electrical signals, optical signals, electric power, etc. between two relatively rotating members by means of a transmission cable which is fixed at both ends individually to the two members and coiled like a spiral spring, and more particularly, to a transmission apparatus of high operating reliability in which possible number of relative rotation of two members can be increased by use of a transmission cable with less turns.
Conventionally, a transmission apparatus which comprises a flexible flat cable (hereinafter referred to as FFC), coiled like a spiral spring, and inner and outer cases containing the same is used for signal transfer between two relatively rotating members, e.g., a steering-shaft-side member of an automobile, as a rotating body, and a steering-column-side member, as a stationary body (Published Unexamined Japanese Utility Model Application No. 62-168581). In this transmission apparatus, two opposite ends of the FFC are fixed individually to the inner and outer cases so that the FFC can be tightened or loosened as the two cases rotate relatively to each other. Thus, signal transmission can be made between the rotating and stationary bodies within a range of rotational angle such that the rotating body, attached to the inner case, for example, can rotate relatively to the stationary body, fitted with the outer case, while tightening or loosening the FFC.
The transmission apparatus of this type is limited to an application such that the rotating body rotates in a reciprocative manner with respect to the stationary body, that is, rotates alternately in either clockwise or counterclockwise directions. Since it includes no sliding-contact portions, this apparatus can constitute a high-reliability transmission line. Meanwhile, the potential frequency or possible number of relative rotation of the inner and outer cases depends on the number of turns of the FFC. In order to increase the possible number of rotations, therefore, the number of turns of the FFC, that is, the FFC length, should be increased. If the number of turns of the FFC is increased, however, the rotatory force of the rotating body cannot be easily transmitted to the whole FFC, so that trouble is liable to be caused such that the whole FFC cannot uniformly move as the rotating body rotates, or the FFC is turned over or bent.
Accordingly, an improved transmission apparatus has conventionally been proposed such that the potential frequency of relative rotation of the inner and outer cases can be increased with less turns of the FFC. In this apparatus, the FFC is formed having a U-turn portion in the middle so that it is wound in one direction on the inner case side and in the opposite direction on the outer case side. In rotating the inner case relatively to the stationary outer case in a direction such that the FFC is wound thereon, according to this proposed apparatus, the U-turn portion of the FFC moves around the inner case, in the same direction as the rotating direction of the inner case. This movement of the U-turn portion cancels half the rotation of the inner case. Also, the rotatory force of the inner case can be satisfactorily transmitted to the whole FFC. Thus, the possible number of rotations can be increased with less turns of the FFC.
If the inner case is rotated in the opposite direction or in a direction such that the FFC is wound off the inner case, however, the FFC slackens, so that the U-turn portion cannot smoothly move in the rotating direction of the inner case. In consequence, the movement of the U-turn portion cannot fulfill its functions to cancel the case rotation and transmit the rotatory force.
Thus, according to the proposed apparatus described above, the movement of the U-turn portion lacks in smoothness, depending on the direction of the case rotation, so that the intended function to increase the rotational frequency of the case with less turns of the FFC cannot be obtained. Also in the proposed apparatus, it is essential for the case to be rotated in a reciprocative manner, that is, the case must inevitably be rotated in a direction such that the U-turn portion cannot smoothly move. Accordingly, the proposed apparatus has not been put into practical use yet.